1. Field of the Invention
The invention relates to a process for preparing 2,2′-biphenol using selenium dioxide.
2. Discussion of the Background
The direct coupling of phenols to give the corresponding biphenol derivatives which are of great industrial interest continues to be a challenge since these reactions are often neither regio- nor chemoselective.
The term “phenols” is used as a generic term in this application and therefore also encompasses substituted phenols.
One possible way of synthesizing these biphenols is by means of electrochemical processes. In this case, carbon electrodes such as graphite, glassy carbon, BDD or transition metals such as platinum are used (cf. F. Stecker, A. Fischer, I. M. Malkowsky, S. R. Waldvogel, A. Kirste, WO 2010139687 A1 and A. Fischer, I. M. Malkowsky, F. Stecker, S. R. Waldvogel, A. Kirste WO 2010023258 A1). A disadvantage of these electrochemical methods is the cost of some of the apparatus, which has to be manufactured specially. Moreover, scale-up to the ton scale, as is typically required in industry, is sometimes very complex and in some cases even impossible.
Direct cross-coupling of unprotected phenol derivatives under conventional organic conditions has been possible only in a few examples to date. For this purpose, usually superstoichiometric amounts of inorganic oxidizing agents such as AlCl3, FeCl3, MnO2, or DDQ, which is organic, are used (cf. G. Sartori, R. Maggi, F. Bigi, M. Grandi, J. Org. Chem. 1993, 58, 7271).
Alternatively, such coupling reactions are conducted in a multistage sequence. In this case, leaving functionalities and often toxic, conjugated transition metal catalysts based on palladium, for example, are used.
A great disadvantage of the abovementioned methods for phenol coupling is the need for dry solvents and for exclusion of air. Both mean a high level of complexity, specifically when the process is to be used on the industrial scale.
Furthermore, the reactions described in the related art often give rise to toxic by-products which have to be removed from the desired product in a complex manner and disposed of at great cost. The increasing scarcity of raw materials (for example boron and bromine) and the rising relevance of environmental protection is increasing the cost of such transformations. Particularly in the case of utilization of multistage syntheses, an exchange of various solvents is necessary, which constitutes a high level of complexity and is an additional cost factor.